This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-089397 filed on Mar. 28, 2000, the entire content of which is incorporated herein by reference.
The present invention relates to a hydraulic piston, and more particularly to a hydraulic piston made of a resin (hereinafter xe2x80x98resin hydraulic pistonxe2x80x99) for a disc brake assembly which is mainly utilized as a wheel brake system of an automobile.
As one of the prior arts related to the present invention, a resin hydraulic piston is disclosed in Japanese Patent Laid-Open Publication 62(1987)-137435.
The above prior art resin hydraulic piston provides the following two advantages: a light-weight structure and a high heat resistance.
Following recent conservation efforts of saving natural resources, minimizing fuel usage, and environmental conservation and so on, components used for an automobile have been required to minimized weight. Since resin is light-weight, the prior art resin hydraulic piston is favorable viewed as a hydraulic piston for a disc brake assembly. Taking a resin hydraulic piston which is filled with glass fibers as an example, tensile strength of the resin hydraulic piston approximately reaches the tensile strength of an aluminum casting-alloy. In addition, the weight of the piston is almost two-third that of an aluminum alloy, which indicates a light-weight structure.
Generally speaking, the frictional heating caused by the frictional engagement between a disc brake pad and a disc rotor is transferred to a brake fluid through a hydraulic piston made of metal (hereinafter xe2x80x98metallic hydraulic pistonxe2x80x99). On the contrary, if the hydraulic piston is made of resin, the high heat resistance of the resin hydraulic piston minimizes the frictional heating transferred to the brake fluid. Therefore, a disc brake provided with the resin hydraulic piston does not cause a vapor lock phenomenon as often as a metallic hydraulic piston does.
However, the hardness of the resin hydraulic piston proves is a disadvantage. Since the resin hydraulic piston is substantially less hard than the metallic hydraulic piston, an outer periphery of the resin hydraulic piston may be easily flawed when the resin hydraulic piston slidably moves in a cylinder bore. In addition, when the resin hydraulic piston is pulled from the cylinder bore at a high air pressure and replaced with a new piston, the resin hydraulic piston sometimes may be cracked due to contact with the cylinder. Flaws on the outer periphery of the resin hydraulic piston may cause the brake fluid to leak, which leads to functional brake defects. Cracks in the resin hydraulic piston also may raise the maintenance cost of the piston.
To overcome the above drawbacks in hardness, which the prior art of the conventional resin hydraulic piston may cause, it is necessary for the prior art hydraulic piston to be formed by a resin cylinder and a metallic cylinder. The present invention prevents flaws and cracks in the resin hydraulic piston and provides a light-weight disc brake having a high reliability.
Accordingly, it is an object of the present invention to provide an improved hydraulic piston which prevents flaws and cracks in the hydraulic piston and provides a light-weight disc brake assembly having a high reliability.
It is a further object of the present invention to minimize the manufacturing cost of the hydraulic piston.
It is yet another object of the present invention to provide various design choices for the hydraulic piston.
According to the present invention, the hydraulic piston includes a metallic cylinder with a bottom at one end and a hollow resin cylinder with an opening at each end disposed in an inner surface of the metallic cylinder. The opening at one end of the hollow resin cylinder is closed by the bottom of the metallic cylinder. According to this structure of the present invention, a main part of the hydraulic piston is formed by the hollow resin cylinder. An outer periphery of the hydraulic piston is coated with metal.
The effects in this invention of the hydraulic piston are as follows: The metal coating causes very few flaws and cracks in the hydraulic piston when the hydraulic piston slidably moves in a cylinder bore. In addition, the present invention takes advantage of a light-weight structure of the hollow resin cylinder.
According to another aspect of the present invention, the hollow resin cylinder includes a main part disposed in the inner surface of the metallic cylinder and a flange portion formed on the main part at the other end of the hollow resin cylinder. An outer diameter of the flange portion is wider than the outer diameter of the main part. An open end of the metallic cylinder is disposed by an outer periphery of the main part of the hollow resin cylinder.
The effects in this structure of the hydraulic piston mentioned above are as follows: An annular groove is defined between the flange portion and the open end of the metallic cylinder. A hermetic seal can be disposed in the annular groove. This structure dispenses with the process to form a seal groove to be disposed with the hermetic seal therein. Therefore, this structure defining the annular groove between the flange portion and the open end of the metallic cylinder is effective to minimize manufacturing costs of the hydraulic piston.
The present invention suggests two choices to form the flange portion, which indicates advantages of the present invention: forming the flange portion integrally with the main part of the hydraulic cylinder and forming the flange portion by a separate member from the main part of the hydraulic cylinder.
According to a further aspect of the present invention, the flange portion is formed integrally with the main part of the hollow resin cylinder. The effect in this structure is as follows: This structure does not require complicated mold shapes and machining processes after forming. Therefore, this structure is effective to minimize the manufacturing costs of the hydraulic piston.
A still further aspect of the present invention is that the flange portion is formed by a separate hollow member which is separate from the main part of the hollow resin cylinder. The flange portion is formed by disposing a ring-shaped cylinder onto the main part at the other end of the hollow resin cylinder where the outer diameter of the main part is uniform. The effect in this structure is as follows: According to the ring-shaped cylinder disposed onto the main part at the other end of the hollow resin cylinder, the flange portion and the main part can be formed from a simple hollow cylindrical material. Therefore, this structure does not necessarily require any mold to form the flange portion.
When the production number of the cylinder is small, the ring-shaped cylinder may be made from a cut resin pipe, which leads to further reduction in cost. In other words, material for the flange portion does not always have to be the same as that for the main part of the hollow resin cylinder. Therefore, this structure allows various design choices for the hydraulic piston.
According to another aspect of the present invention, a disc portion made of resin (hereinafter xe2x80x98resin disc portionxe2x80x99) is disposed between the opening at the one end of the hollow resin cylinder and the bottom of the metallic cylinder at the inner surface. The effects in this structure are as follows: This structure improves the strength of the bottom portion of the hydraulic cylinder to which brake pressure is to be applied. Further, as far as the variety in thickness of the resin disc portion and the hollow resin cylinder, the hydraulic piston is able to be applied for any specification from low pressure use to high pressure use. Therefore, this structure allows various design choices.
This present invention suggests two choices to shape the bottom of the metallic cylinder, which indicates advantages of this present invention: a concave shape and a convex shape.
According to a further aspect of the present invention, the bottom of the metallic cylinder is of a concave shape which bend inwardly towards an inner surface of the hollow resin cylinder. In other words, the bottom of the metallic cylinder is of a concave shape which projects inwardly from an outside of the hydraulic piston. The effect in this structure is as follows: This concave structure serves as a part of hydraulic pressure chamber defined by the hydraulic piston and the cylinder bore and assures the volume of the hydraulic pressure chamber.
A still further aspect of the present invention includes the structure that the bottom of the metallic cylinder is of a convex shape which projects away from the inner surface of the hollow resin cylinder. In other words, the bottom of the metallic cylinder is of a convex shape which projects outwardly from an inside of the hydraulic piston. Compression stress is generated at the bottom of the metallic cylinder when the brake hydraulic pressure is applied thereto. Therefore, this structure greatly improves the hydraulic piston in strength.
Thus the present invention has the following advantages: upon the hydraulic piston being slidably moved in the cylinder bore, the hydraulic piston formed by the hollow resin cylinder with an opening at each end and the metallic cylinder with a bottom at one end prevents flaws and cracks in the hydraulic piston. Thus, this present invention improves the hydraulic piston in strength, weight and cost with a high reliability. In addition, this present invention allows various design choices.